ZURICH, July 15 (Reuters) - Novartis said it would test two experimental Alzheimer's drugs on people with a genetic risk of developing dementia, aiming to gauge whether the treatments can prevent or delay symptoms of the memory-robbing disease.

In collaboration with the Banner Alzheimer's Institute, the Swiss company will study two therapies in cognitively healthy people who are at risk of developing a build-up in the brain of amyloid protein, a toxic protein which is believed to cause Alzheimer's.

Currently approved medications only treat symptoms and there are no licensed drugs that can slow the progression of the disease, which gradually robs patients of their ability to think and care for themselves.

Dementia - of which Alzheimer's disease is the most common form - already affects 44 million people worldwide and this total is set to reach 135 million by 2050, according to Alzheimer's Disease International, a non-profit campaign group.

One of Novartis's treatments is an immunotherapy, an injectable medicine in Phase II clinical trials which works by stimulating the immune system to produce natural antibodies that attack amyloid.

The second treatment is a so-called BACE inhibitor drug, an oral pill which is about to enter Phase I trials. This class of treatments work by blocking an enzyme called beta secretase that is involved in production of beta-amyloid.

The trial will involve more than 1,300 cognitively healthy patients aged between 60 and 75 and is planned to start this year.

The patients have two genetic copies of apolipoprotein E episilon 4 (APOE4) allele, a gene that contains instructions for making a protein that carries cholesterol and is a well-known risk factor for Alzheimer's. People who get the gene from both parents have a 10-fold risk of developing Alzheimer's.

Drugmakers have been working for years to develop so-called disease-modifying drugs, but it is proving an uphill battle. No new therapies have been approved to treat Alzheimer's in a decade, according to a recent study from researchers at the Cleveland Clinic.

Many scientists increasingly believe the best hope is testing drugs much earlier in the process, before patients' brains are wrecked by Alzheimer's. (Editing by David Holmes)